1. Field of the Invention
The invention relates to an element impedance detection apparatus and method for an oxygen sensor. More particularly, the invention relates to an element impedance detection apparatus and method which is applicable to an oxygen sensor having an element impedance that changes depending on temperature characteristics, and which is suitable for measurement of an element impedance thereof.
2. Description of the Related Art
Apparatuses are known that detect element impedance of an oxygen sensor, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 2000-028575. In the disclosed apparatus, a constant impressed voltage V0 is applied to the oxygen sensor; when element impedance is detected, a function of the apparatus changes the impressed voltage V0 from a reference voltage to a sweep voltage. When the impressed voltage V0 changes by a voltage change ΔV0, a current I flowing in the oxygen sensor changes by current change ΔI, which corresponds with an element impedance Rs. It should be noted that in the above-described apparatus, the element impedance of the oxygen sensor is calculated based on the voltage change ΔV0 generated by changing the impressed voltage V0 to the sweep voltage and the current change ΔI.
The apparatus described above is configured such that, when the impressed voltage V0 is changed from the reference voltage to the sweep voltage, a time constant of the change is normally constant. Further, in this apparatus, normally, both terminal voltages of the oxygen sensor are changed to the sweep voltage when element impedance is measured, regardless of the magnitude of element impedance.
Generally, element impedance of an oxygen sensor exhibits temperature characteristics, and the element impedance value changes within a substantial range. In the above-described apparatus, when element impedance becomes a sufficiently large value, even if both terminal voltages of the oxygen sensor are changed to the sweep voltage, the current I flowing in the oxygen sensor does not become excessive. However, with the above-described apparatus, in a state where element impedance is sufficiently small, when the impressed voltage V0 applied to the oxygen sensor is changed to the sweep voltage, the current I flowing in the oxygen sensor sometimes becomes excessive. In this way, a characteristic feature of the above-described apparatus is that excessive electric power is supplied to the oxygen sensor when detecting element impedance.